Magnetic resonance apparatuses are being used ever more frequently to solve various problems, imaging methods of magnetic resonance apparatuses being based on the fact that high frequency pulses (HF pulses) excite the nuclear spins, present in the human body, of hydrogen protons. These nuclear spins excited by high frequency pulses relax back into their original position, it being possible to detect the change in magnetization with the aid of various coils.
Attempts are also being made to use magnetic resonance apparatuses with a higher magnetic basic field strength B0, since it is possible thereby to amplify the recorded signal and enhance the spatial resolution. However, the use of magnets with a higher field strength also leads to the problem of fluctuation in the signal intensity imaged in the MR image. These intensity fluctuations are based chiefly on the fact that the irradiated HF field interacts with the body being examined.
Very uniform high frequency fields can be produced in vacuo or in nonconducting materials. These uniform high frequency fields are necessary for the uniform excitation of the nuclear spins present in the examination object. However, biological materials have dielectric properties that result in the effective high frequency field becoming a function of frequency and nonuniform such that it is no longer possible for the nuclear spins in a predetermined slice of the examination object to be excited homogeneously (bias field). This nonuniformly produced transverse magnetization is detected by the receiving coils and leads to intensity fluctuations and even to signal failures in the MR image such that it is no longer possible to diagnose in this area of the MR image. The frequency of the irradiated HF pulses is directly proportional to the basic field strength, and so these HF-induced inhomogeneities occur in an amplified fashion given strong B0 fields.
U.S. Pat. No. 6,208,138 B1 describes a correction method for eliminating inhomogeneities in an MR image, a logarithm being applied to the MR image, the inhomogeneities being removed, and the image subsequently being resubjected to an exponential calculation. This type of image processing with the use of homomorphic filters is based on the fact that the inhomogeneity is superposed over the image as a spatially dependent multiplier, that is to say the original intensity is multiplied by an inhomogeneity profile. Owing to the application of a logarithm, this multiplication is converted into an addition and can easily be removed or filtered out from the image. The signal intensities are exponentiated after the removal of the inhomogeneity component in order to cancel once again the previously applied logarithm step.